xy is a separate symbol, not the product of the symbols x and y. Try
x*y
Bob Hanlon
---- Srikanth <skt at xdtech.com> wrote:
=============
Hi
I am using mathematica to fit data to a surface. My data is in a
matrix form:
Data1={{1.1, 7, 8.765}, {1.1, 7.5, 10.6481},.....,{1.9, 10.5,
26.1048}}
I want to get a form z=f(x,y) that fits this data, with x and y being
my first and second element of each vector. I expect the solution to
be a linear combination of x, y, xy and some constant term.
So I tried
Fit[Data1, {1, x, y, xy}, {x, y}]
and I get an error message:
Fit::fitm: Unable to solve for the fit parameters; the design matrix
is nonrectangular, non-numerical, or could not be inverted.
I'm not sure what it meant by non-numerical (for example Data1[[1,1]]
gives 1.1 as expected). The data is rectangular and well defined (by
which I mean I never have cases like {x, , z} with y missing or
anything absurd like that). It cannot be inverted of course, but that
didn't seem to be a problem from the example 9 given here:
http://reference.wolfram.com/mathematica/tutorial/CurveFitting.html
Any suggestions on what I need to do?
Thanks for your help
Srikanth